1. Field of the Invention
The present invention relates to an optical printer head using a plurality of LED (light-emitting diode) arrays.
2. Description of the Prior Art
Recently, a variety of optical printer heads using LED are widely made available for industrial uses. Of these, in particular, the one so-called one-line type optical printer head using a plurality of linearly-aligned LEDs along the total length of the main scanning line such as the one proposed by the U.S. Pat. No. 4,605,944 shares the majority. The optical printer head cited above comprises a plurality of linearly-aligned LED arrays being wired thereto and having emitter elements being aligned on the surface. The optical printer head having the above constitution is particularly useful for satisfying the needs for finer resolution for example, a resolution of more than 8 dots/mm is obtained. Since a number of emitter elements are arranged in a specific density equal to the print resolution, and yet, since these emitter elements are turned on in response to the print data, they generate substantial amount of heat. To improve the radiation efficiency, as cited in the U.S. Pat. No. 4,524,372, an art has been proposed for directly fixing LEDs onto a metal base plate made from aluminum for example. However, the proposed art still has some problems to solve. Concretely, thermally elongaged base plate may cause the LED array to eventually be disengaged from it or the expanded intervals of the juncture of adjoining LED arrays may cause the printing density to become loose. Furthermore, densely-distributed metallic leads connected to LED arrays may distort themselves to eventually cause either disconnection or poor contact to take place.
Any conventional optical printer head is provided with a typical constitution in which light is transmitted from the LED array to the printing object by applying either optical fibers or prism. Since it is not always necessary for the conventional optical printer head to densely arrange the LED arrays in the direction of aligned emitter elements wiring means made of insulated sheet with fine metal leads supported thereon has been conventionally made available for wiring the LED arrays. Due to relatively easy handling characteristic of this means material mentioned above, it may desirably be applied to one-line type optical printer head, i.e., to the one which densely arranges a plurality of LED arrays in the direction of the aligned emitter elements. However, this cannot be implemented due to the presence of some critical problems described below.
Although any conventional one-line type optical printer head has a number of emitter elements and electrodes which are individually attached to each of these emitter elements it is extremely difficult for this optical printer head to provide a metal lump on each of these densely-disposed electrodes. In particular, any lump having a substantial height cannot be formed in those narrow regions on the compound semiconductor of an LED array. Even if such a tall lump were provided in a relatively wide region, short circuit will easily happens between adjoining lump. To compensate for this, there is an idea for providing metal lumps which are to be formed on the fine leads of the wiring means. However, since these metal leads are too fine, and yet, since the metal lump should be set to both ends of metal leads by applying a microscope, when setting a metal lump to the other end after setting it to an end of the metal lead and also when pressing an end of the metal lead against the LED array for connection, heat generated by the pressure transmits itself through the fine metal leads, thus often causing the metal lump at the other end of the lead to fall down itself from the fine metal leads.
Furthermore, if the metal lump is installed onto a individual electrode like the one made of aluminum for example without support on a compound semi-conductor, the metal lump is easily flow or roll itself from the electrode due to thermal effect, or even if the metal lump is insufficiently connected to the individual electrode, the metal lump is easily be stripped off from the electrode due to the presence of 2 through 10mA of current flowing through the emitter elements. As a result, any conventional optical printer head cannot be effectively distribute fine metal leads with satisfactory productivity.
Likewise, since it is necessary to provide the LEDs with specific intervals at junctures being equal to those intervals of other portions, fine metal leads should be distributed in positions very close to each other without allowing both the cutting and connection work to be done smoothly. In particular, fine metal leads at edge portions easily become useless. For example, only a maximum of 125 micrometers (corresponding to 8 pieces per millimeter) of the interval can be generated even if wires are alternately extracted to both sides of the emitter elements arranged by 16 dots per millimeter. Likewise, even if these wires are drawn to specific positions closest to the center of the LED array, only about 500 micrometers of intervals can be provided at the junctures. On the other hand, since each fine metal lead has 50 micrometer of width for example, when cutting the insulated sheet into blocks at each LED array, these leads are easily damaged or stripped off from the insulated sheet, and thus the above method is not desirable for use.
In the light of rate of the non-defective and the characteristic of the operation, manufacturers cannot use such an LED array and leads having length corresponding to the scanning length, i.e., the full length of the printing line. When a plurality of LED arrays or leads are employed, adequate measures should be applied to the juncture portions. It is necessary for the juncture portions to be provided specific intervals between emitter elements being equal to those intervals between other portions, and in addition, fine metal leads corresponding to emitter elements should also be provided. The insulated sheet supporting fine metal leads projects itself from the row of metal leads, and in addition, wiring means may overlap themselves at the juncture portion to result in the formation of cubical shape. This easily allows stress to be applied to the wire-connected portion, thus eventually resulting in the poor connection or the inability to properly repair the faulty portion.
Generally, any conventional LED array is disposed in the lengthwise direction, while these LEDs are provided with the LED drivers which are aligned in the lengthwise direction. Thus, when aligning the LED arrays after connecting them to the drivers through fine metal leads, the leads connected to the drivers may obstruct the alignment work, or these wires may improperly be connected to each other. Conversely, when the alignment of the LED array precedes, the wiring means handling work become extremely complex. Consequently, wither of these conventional procedures has eventually resulted in inconvent processing work and low rate of the non-defective.